1. Field of the Invention
The present invention relates to a method and an apparatus for inspecting a measured object on which an image is formed, an image forming apparatus including the image inspecting apparatus, and a computer-readable recording medium storing an image inspecting program.
2. Description of the Related Art
In an electrophotographic image forming apparatus, a toner image formed on a sheet of paper as an image carrying medium is fixed onto the sheet by melting the toner by a thermal fusing unit in a step called “fusing”. Thus, the gloss of the fused toner image may be varied depending on the amount of toner attached onto the sheet. In such an image forming apparatus, the type of the sheet as an output image base may be varied. For example, the fusing condition of the sheet with respect to the toner varies depending on the thermal capacity of the sheet, resulting in variations in the gloss of the fused toner image. Also, when the gloss of the sheet itself is varied, the gloss of the toner image formed on the sheet varies.
Various attempts have been made to improve image quality in terms of gloss. For example, the gloss of the toner image may be varied by controlling the fusing unit in accordance with user demand. A transparent toner may be used to actively impart gloss to the output image. Further, an image may be formed on a high-gloss sheet.
However, even when the toner is attached onto the sheet accurately with an intended toner amount distribution, the fused condition of the toner on the sheet may become uneven, resulting in the appearance of lines, due to a problem in the fusing unit. Specifically, visible lines called “fusing lines” may appear when the fused sheet is observed at an angle, although there is little variation in the density distribution. Such an image is determined to be defective.
As an output image inspecting apparatus, an image inspecting apparatus may be considered that is capable of inspecting the output image from an electrophotographic image forming apparatus on an on-demand basis. Desirably, such an image inspecting apparatus is capable of measuring not just the density distribution of an entire area of an output image but also its gloss distribution.
The gloss of an object may be measured by illuminating the object with illuminating light at a certain incident angle and then measuring the intensity of specular light reflected from the measured object. In this case, because the specular light is measured, the incident angle and the reflected angle are the same, and these angles may be set depending on the measured object. The density of an area of the object may be measured by illuminating the area of the measured object with illuminating light from a direction such that no specular reflection is caused, and measuring the intensity of diffused reflected light reflected from the area.
A technology for inspecting a gloss distribution is discussed in Japanese Laid-open Patent Publication No. 2005-277678. In this technology, gloss inspection is performed on the assumption that, when image data read from a read area in the direction of diffused reflected light provide substantially the same value, the read area has substantially the same density, and that, if there is gloss abnormality in the read area, a difference will be caused in the image output of the read area based on the image data read in the direction of specular light. Namely, the technology assumes that the gloss is the same when the density is the same.
However, even when the image density is the same, the amount of toner attached (“attached toner amount”) may vary depending on the density of the colorant of the toner or the toner size. It is also known that the gloss of a fused image may vary depending on the toner attached amount. Further, when gray levels of an image, such as a natural image, are formed by superposition of halftone dot images of plural colors of toner, such as cyan, magenta, and yellow, each of the image-forming colors has a density distribution. Thus, the gloss distribution of the overall image varies depending on the toner attached amount regardless of the image-forming color.
Specifically, rather than the toner attached amount of each individual color, a total value of the attached amounts of the plural colors of toners is important. This means that, even when the image density is the same, the toner attached amount may vary, and therefore the gloss may also vary. Thus, the relationship that dictates that when the image density is the same, the gloss is the same does not hold. In other words, a gloss distribution cannot be accurately inspected solely based on the relationship between image density and gloss.